I'm not 100% sure whether this should be possible, but it would appear to be within the rules.
I tried to set a board up that has the Augurs (indicated by the Herb Gatherer) as the card for the Way of the Mouse.
Although the Herb Gatherer appears on the list to select, if I select it and attempt to start the game, red text appears saying this is not a kingdom card. Please see attached screenshot.
Split pile cards from Empires (eg Catapult) can be selected, but testing with the other Allies cards it seems non of the Allies split cards can be used in this way.
Is this a bug, or have I missed a rule somewhere?
Thanks
Yes, this is unfortunately a bug that has crept in at some time.
As an additional clarification, Way of the Mouse only contains one card, not an entire pile. So you can put Herb Gatherer there, but that's it, it doesn't "also contain" the other cards from the Augur pile.
Thanks for the reply Ingix, yes that makes sense that it would only be Herb Gatherer and not the whole pile selected for Way of the Mouse.
Maybe related -- I just had a game where Way of the Mouse had Battle Plan as its card, even though Battle Plan (and the other clashes) were a kingdom pile.